The effect of cooling rate on plastic deformation of Zr-based bulk metallic glasses has been studied.The specimens with the diameters of 3 mm,4 mm and 6 mm cut from the same ladder-shaped Zr55Al10Ni5Cu30 BMG rod were used in the present investigation.The experimental results indicated that the different plastic strains were induced for the three type compression specimens.The compressive plastic strain increased with the decreasing diameter for the as-cast specimens,which means that the compressive plastic strain increased with the increasing cooling rate.The reason of the difference is attributed to the free volume existed in the BMGs during the rapid solidification,and the larger free volume was contained in the specimen fabricated with the higher cooling rate.It has been found that the prominent serrations and multiple shear bands appeared during the compressive deformation tests of the specimen with smaller diameter of 3 mm,indicating that the specimen with smaller diameter exhibits better plasticity due to more free volume content.